DOI: 10.1002/chem.201304404
Communication
& Hybrid Materials
Synthesis of Nanoporous Carbon–Cobalt-Oxide Hybrid Electrocatalysts by Thermal Conversion of Metal–Organic Frameworks Watcharop Chaikittisilp,[a] Nagy L. Torad,[a, b] Cuiling Li,[a] Masataka Imura,[c] Norihiro Suzuki,[d] Shinsuke Ishihara,[a] Katsuhiko Ariga,*[a, e] and Yusuke Yamauchi*[a, b, e] Abstract: Nanoporous carbon–cobalt-oxide hybrid materials are prepared by a simple, two-step, thermal conversion of a cobalt-based metal–organic framework (zeolitic imidazolate framework-9, ZIF-9). ZIF-9 is carbonized in an inert atmosphere to form nanoporous carbon–metallic-cobalt materials, followed by the subsequent thermal oxidation in air, yielding nanoporous carbon–cobalt-oxide hybrids. The resulting hybrid materials are evaluated as electrocatalysts for the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER) in a KOH electrolyte solution. The hybrid materials exhibit similar catalytic activity in the ORR to the benchmark, commercial, Pt/carbon black catalyst, and show better catalytic activity for the OER than the Pt-based catalyst.
Tremendous efforts in the development of nanoporous materials, including nanoporous carbon, have been made in recent years to create novel materials with desired structures and [a] Dr. W. Chaikittisilp, Dr. N. L. Torad, Dr. C. Li, Dr. S. Ishihara, Prof. Dr. K. Ariga, Prof. Dr. Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) Homepage: http:www.yamauchi-labo.com E-mail: [email protected] [email protected] [b] Dr. N. L. Torad, Prof. Dr. Y. Yamauchi Faculty of Science and Engineering, Waseda University 3–4–1 Okubo, Shinjuku, Tokyo 169-8555 (Japan) [c] Dr. M. Imura Optical and Electronic Materials Unit National Institute for Materials Science (NIMS) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) [d] Dr. N. Suzuki International Center for Young Scientists (ICYS) National Institute for Materials Science (NIMS) 1–2–1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) [e] Prof. Dr. K. Ariga, Prof. Dr. Y. Yamauchi Precursory Research for Embryonic Science and Technology (PRESTO) & Core Research for Evolutional Science and Technology (CREST) Japan Science and Technology Agency (JST) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304404. Chem. Eur. J. 2014, 20, 4217 – 4221
properties.[1] As a new class of nanoporous materials, metal–organic frameworks (MOFs) or porous coordination polymers represent fascinating properties and functions with modularly tunable structures assembled from metal cations and organic ligands.[2] In addition to the promising applications of MOFs, such as gas storage and separation, catalysis, sensing, and drug delivery, they have been demonstrated recently to be useful precursors for the synthesis of metal oxides with controlled particle sizes and morphologies,[3–5] and nanoporous carbon.[6–8] Because of the large variation of organic moieties in MOFs, highly nanoporous carbon, with excellent properties in gas adsorption, electrochemical capacitance, and sensing, can be achieved by thermal carbonization of several MOFs that serve as precursors or sacrificial templates in inert atmospheres. The procedure is followed by chemical treatments to remove any inorganic residuals from the carbon networks.[6–8] Xu et al. have reported MOF-derived nanoporous carbon for the first time.[7] Interestingly, MOF-derived nanoporous carbon can have exceptionally high surface areas and/or very uniform pore-size distributions.[6] However, the direct preparation of carbon–metaloxide hybrid materials from a MOF as a single precursor is still rare.[9] Recently, carbon–metal-oxide hybrid materials have become of particular interest for electrochemical applications, such as fuel cells and energy-storage devices. Oxygen-reduction reactions (ORRs) are important cathode reactions in fuel cells, metal–air batteries, and chlor-alkali electrolysis.[10] So far, platinum and alloys thereof have been found to be the most active electrocatalyst for ORRs. However, the cathode ORR is much slower than the anode oxidation reaction, thereby requiring a high loading of expensive Pt-based catalysts to accelerate the ORR.[11] This limits the commercialization of fuel cells, particularly in the automotive industry. In the course of searching for active and robust nonprecious metal catalysts for the ORR, metal–nitrogen complexes (where the metal is cobalt or iron) on carbon matrixes with M N4 (M = Co or Fe) catalytic active sites were found to show excellent performance for the ORR in acidic media.[12] Carbon-supported metal oxides, particularly cobalt-based oxides, have been found to be active ORR catalysts in alkali media.[13–15] Strong coupling between metal oxides and carbon matrixes has been suggested to enhance the electrocatalytic activity. Among several strategies, the direct nucleation, growth, and anchoring of metal oxides on the oxidized carbon atoms seem to be one of the most fasci-
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Communication nating ways to yield efficient carbon–metal-oxide hybrid electrocatalysts.[13] To the best of our knowledge, there are only a few reports in the literature on the utilization of MOFs, either as precursors[16] or as supports,[12d,e] for the preparation of electrocatalysts for the ORR in acidic media. In the former case, where MOFs were used as precursors, the direct thermal treatment of MOFs containing Co N bonds resulted in continuously connected carbon networks, thereby providing electrically conductive support for electrocatalysts.[16] In the latter reports, MOFs served as microporous hosts for catalyst precursors, that is, iron cations and bidentate N-donor ligands.[12d,e] Alternative to the above reports, we describe herein for the first time the preparation of nanoporous carbon–cobalt-oxide hybrid materials for the ORR in alkali media by direct conversion of a MOF as a single precursor. We anticipated that the resulting hybrids would possess a strong coupling between cobalt oxide and the carbon matrixes. Zeolitic imidazolate framework-9 (ZIF-9), which possesses a sodalite topology with hexagonal symmetry, is built from corner-sharing tetrahedral CoN4 units, in which the coordination bonds between the Co2 + cations and benzimidazolate anions is among the most stable for N-donor ligands.[17] In this study, ZIF-9 was selected as the cobalt-based MOF precursor due to its good thermal stability and high contents of carbon and nitrogen. ZIF-9 was solvothermally synthesized from cobalt nitrate and benzimidazole (see the Supporting Information, Figure S1). Conversion of ZIF-9 into carbon–cobalt-oxide hybrid materials was performed by two-step thermal treatments in an inert atmosphere and air, respectively (Figure 1). The resulting hybrid materials were designated as Z9-x-y, where x and y stand for the thermal treatment temperatures in the first and the second steps, respectively. Figure 2 a shows powder XRD patterns of the obtained hybrid materials after the first thermal treatment, indicating the presence of metallic cobalt in the materials. The XRD peaks become sharper as the treatment temperature increases, indicating an increase in the Co crystallite sizes. TEM images show that the Co nanoparticles are uniformly distributed over the entire area and that the sizes gradually increase with increas-
Figure 2. Powder XRD patterns (a,c) and nitrogen adsorption–desorption isotherms (b,d) of the samples obtained after the first thermal treatment in a nitrogen atmosphere (a,b) and after the subsequent treatment in air (c,d). The adsorption and desorption isotherms are presented as solid and open symbols, respectively.
ing treatment temperatures (see the Supporting Information, Figure S2). As assessed by nitrogen adsorption–desorption measurements (Figure 2 b), the materials are micro- and mesoporous with apparent specific Brunauer-Emmett-Teller (BET) surface areas of 415, 370, and 330 m2 g 1 for Z9-700-0, Z9-8000, and Z9-900-0 samples, respectively. Their total pore volumes (at P/Po = 0.99) are nearly identical, about 0.4 cm3 g 1. Thermogravimetric analysis (TGA) of the Z9-900-0 sample in air suggests that the cobalt particles undergo thermal oxidation as the sample weight increases with the maximum weight being at about 300 8C (see the Supporting Information, Figure S3). Figure 1. Schematic representation of the formation of carbon–cobalt-oxide hybrid materials through the twoAs a result, we first attempted to step thermal treatment of ZIF-9. Chem. Eur. J. 2014, 20, 4217 – 4221
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Communication oxidize the metallic cobalt particles in air at 300 8C. The XRD patterns of the samples obtained after the second thermal treatment clearly indicate the conversion of metallic cobalt to cobalt oxide (Co3O4, see the Supporting Information, Figure S4 a). The resulting hybrid materials, however, loose the porosities when the BET surface areas decrease to 50–70 m2 g 1. The sharp nitrogen sorption isotherms at high relative pressures (P/Po > 0.9) suggest the presence of large interparticular pores, due to the cobalt-oxide nanoparticle aggregates (see the Supporting Information, Figure S4 b). TGA results show that the residual weights (after 600 8C in air) are about 97, 94, and 82 wt % for the Z9-700-300, Z9-800-300, and Z9-900-300 samples, respectively, suggesting that the carbon networks decompose during the oxidative thermal treatments. TEM images of the Z9-x-300 samples (see the Supporting Information, Figure S5) also show that these samples are made of nanoparticle aggregates with interparticular pores. We hypothesized that the carbon networks are retained after oxidative thermal treatments at milder conditions. Therefore, the Z9-x-0 samples were subjected to the oxidative thermal treatment at 250 8C in air. As shown in Figure 2 c, the obtained samples clearly exhibit XRD peaks that arise from Co3O4 with some remaining cobalt metals. The coexistence of metallic cobalt was more markedly observed for the Z9-900-250 sample. In addition, the formation of CoO can be markedly seen in the case of the Z9-900-250 sample. As depicted in Figure 2 d, very interestingly, the nitrogen sorption isotherms indicate that after thermal treatment at 250 8C, the Z9-900-250 sample keeps the micro- and mesoporous structures, whereas the nanoporous carbon network of the Z9-700-250 and Z9-800-250 samples decompose, likely yielding the aggregates of cobalt-oxide nanoparticles, similar to the Z9-x-300 samples (see the Supporting Information, Figure S5). TEM images (see the Supporting Information, Figure S6) and the corresponding elemental mapping images (see the Supporting Information, Figure S7 and S8) also support the above claim. These results suggest that carbonization at 900 8C yields more thermally stable carbon networks than those carbonized at lower temperatures. The BET surface areas of Z9700-250, Z9-800-250, and Z9-900-250 were calculated to be 75, 100, and 280 m2 g 1, respectively. Total pore volumes (at P/Po = 0.99) are 0.34, 0.45, and 0.36 cm3 g 1, respectively. The TGA residual weights (after 600 8C in air) of Z9-700-250, Z9-800-250, and Z9-900-250 samples are about 90, 68, and 54 wt %, respectively, further supporting the retained carbon network in the Z9-900-250 sample. In addition, the TGA result suggests that a large amount of carbon still remains in the Z9-800-250 sample, although the porous carbon network is collapsed. Elemental analysis showed that our hybrid materials contain about 10 wt % of nitrogen. We expected that these nitrogendoped carbon networks should promote the ORR catalytic activities as the nitrogen-doped carbons were reported to catalyze the ORR and provide strong coupling between carbon networks and metal-oxide active species.[18] Further decrease of the oxidation temperature to 200 8C results in carbon hybrid materials with only small fractions of oxidized metallic cobalt. Chem. Eur. J. 2014, 20, 4217 – 4221
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The ORR catalytic activities of the resulting carbon–cobaltoxide hybrid materials were evaluated in an aqueous KOH (0.1 m) electrolyte solution through a rotating-disk electrode (RDE) method. RDE measurements were carried out to evaluate the ORR activity and kinetics in the O2-saturated KOH solution. Figure 3 a shows the ORR polarization curves recorded at a rotation rate of 2000 rpm with a scan rate of 5 mV s 1. Surprisingly, the Z9-800-250 sample exhibits better catalytic activity than the Z9-900-250 sample, although the Z9-800-250 sample has lower surface areas. The copresence of CoO and Co3O4 in Z9900-250 is not thought to give any negative effects on the ORR, because CoO-carbon hybrids and their Co3O4-carbon counterparts exhibit very similar, ORR half-wave potentials.[13d, 14c] The outperformance of Z9-800-250 over Z9-900-250 can therefore be attributed to the more complete conversion of metallic cobalt to cobalt oxide and the smaller sizes of cobalt oxide present in the carbon matrixes. Our previous report showed that ZIF-derived carbons obtained at 800– 1000 8C exhibit similar Raman spectra (i.e., similar relative ratios of the G band to the D band) and capacitances.[6c] The Z9-800-250 sample shows a half-wave potential that is about 55 mV more negative than that of the benchmark commercial Pt/carbon black (Pt/CB, 20 %) catalyst under the same conditions and outperformed the Pt/CB catalyst in the term of ORR current density at medium overpotentials; this is consistent with previous reports on similar materials.[13]
Figure 3. a) ORR polarization curves of RDE modified with Z9-700-250, Z9800-250, Z9-900-250, and Pt/CB. The plots are obtained in O2-saturated KOH (0.1 m) at a rotation rate of 2000 rpm with a scan rate of 5 mV s 1. b) ORR polarization curves of Z9-800-250 at different rotation rates in O2-saturated KOH (0.1 m) with a scan rate of 5 mV s 1. c) K–L plots of Z9-800-250, n represents the electron-transfer numbers. d) Polarization curves of glassy carbon electrodes modified with Z9-700-250, Z9-800-250, Z9-900-250, and Pt/CB in KOH (0.1 m).
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Communication To determine the ORR electron-transfer pathway, RDE measurements were performed at different rotation rates, giving the increasing current densities with increasing rotation rates (Figure 3 b). The electron-transfer numbers (n) were calculated from the slope of the Koutecky–Levich (K–L) plots (J 1 vs. w 1/2) on the basis of the K–L equation. Figure 3 c shows the K–L plot for Z9-800-250, which gives the electron transfer numbers of about 4, suggesting that the Z9-800-250 hybrid catalyzes the ORR mainly through a four-electron process. For comparison, the K–L plot at different potentials for commercial Pt/ CB is shown in the Supporting Information, Figure S9 a. Z9700-250 and Z9-900-250 exhibit slightly lower electron transfer numbers (see the Supporting Information, Figure S9 b). As expected, the Z9-x-300 materials exhibit poorer ORR activities with much more negative half-wave potentials than the Pt/CB catalyst (see the Supporting Information, Figure S10 a). The K–L plots shown in Figure S10 b–d, in the Supporting Information, give electron transfer numbers of 2.2, about 3.6, and about 3.5 for Z9-700-300, Z9-800-300, and Z9-900-300, respectively. Although the Z9-x-300 samples show more complete oxidation of metallic cobalt to cobalt oxide than the Z9-x-250 materials, the ORR activities of the Z9-x-300 samples are poorer; this is likely a result of the decomposition of the carbon matrixes. The OER or water oxidation, the reverse reaction of the ORR, is an anode reaction in an electrolysis cells, a half-cell reaction of water splitting (solar-fuel production) and is involved in the recharging process of metal-air batteries.[13a, 19] A bifunctional catalyst that can catalyze both the ORR and the OER is of particular importance as it can be employed in unitized regenerative fuel cells (URFCs), an energy-storage system for uninterrupted power supplies, solar-powered aircraft, satellites and microspacecrafts. URFCs can work as fuel cells and reversely as water electrolyzers to produce hydrogen and oxygen, with the ability to be coupled to intermittent renewable energy, such as wind or solar, to peak-shift electricity to the grid.[20] The resulting carbon–cobalt-oxide hybrid materials were therefore tested for the OER in KOH (0.1 m). As depicted in Figure 3 d, the Z9-700-250 and Z9-800-250 samples exhibit more negative OER onset potentials with higher current densities than Z9-900-250 and Pt/CB catalysts. These results indicate that the Z9-800-250 hybrid can act as an efficient electrocatalyst for both the ORR and the OER and hence may be utilized in the URFCs system. In summary, a direct, two-step thermal conversion of cobaltbased MOFs was performed in an inert atmosphere and in air, yielding nanoporous nitrogen-doped carbon–cobalt-oxide hybrid materials. Metallic cobalt nanoparticles embedded in the nanoporous carbon networks were achieved after the first thermal treatment (carbonization); subsequently, such cobalt nanoparticles were converted to cobalt oxides during the second thermal treatment (oxidation). These hybrid materials exhibited excellent catalytic activities for both oxygen reduction and evolution reactions, and accordingly may be a candidate catalyst for fuel-cells applications. Our simple thermal treatment is anticipated to be applicable to other structures of MOFs and the electrocatalytic activity of the resulting hybrids could be improved by fine tuning of crystallite size and morChem. Eur. J. 2014, 20, 4217 – 4221
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phology of the MOF precursors as well as the thermal-treatment conditions.
Experimental Section Synthesis of hybrid materials Carbon–cobalt-oxide hybrid materials were prepared by a two-step thermal treatment of ZIF-9. In the first step, well-ground ZIF-9 (0.5 g) was homogeneously dispersed in a ceramic boat that was then put into a tube furnace. The sample was exposed to a flow of nitrogen (ca. 45 mL min 1) at room temperature for about an hour and afterward the furnace was heated to the target carbonization temperature (700, 800, or 900 8C) at a heating rate of 5 8C min 1 with an intermediate step at 150 8C for 2 h. After reaching the target temperature, it was kept constant for 5 h and then cooled to room temperature naturally. The resultant black powder was collected for further uses. In the second step, the obtained black powder was thermally treated in air in a muffle oven. The sample was homogeneously dispersed in a ceramic disk. The ceramic disk was heated at the desired temperatures (250 or 300 8C) for 90 min with a heating rate of 1 8C min 1. The obtained product was collected for further characterizations.
Electrochemical test Voltammograms were obtained from an ALS/CHI 842BZ electrochemical analyzer at ambient temperature with a scan rate of 5 mV s 1. A platinum wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. A glassy carbon RDE, the working electrode, was coated with a catalyst ink (0.25 mg cm 2). The catalyst ink was prepared by dispersing catalyst (3 mg) in a Nafion water/ethanol solution (1 mL), which was prepared by adding Nafion (400 mg, 5 wt %) into a mixture of ethanol (12 mL) and water (8 mL). RDE measurements were conducted with a three-electrode electrochemical cell at different rotation rates while the electrolyte was continuously bubbled with oxygen. Before the RDE experiments, the aqueous KOH (0.1 m) electrolyte solution was bubbled with oxygen for at least an hour.
Keywords: cobalt oxide · electrocatalysts · mesoporous carbon · metal–organic frameworks · nanoporous carbon
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Received: November 10, 2013 Published online on March 12, 2014
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& Hybrid Materials
Synthesis of Nanoporous Carbon–Cobalt-Oxide Hybrid Electrocatalysts by Thermal Conversion of Metal–Organic Frameworks Watcharop Chaikittisilp,[a] Nagy L. Torad,[a, b] Cuiling Li,[a] Masataka Imura,[c] Norihiro Suzuki,[d] Shinsuke Ishihara,[a] Katsuhiko Ariga,*[a, e] and Yusuke Yamauchi*[a, b, e] Abstract: Nanoporous carbon–cobalt-oxide hybrid materials are prepared by a simple, two-step, thermal conversion of a cobalt-based metal–organic framework (zeolitic imidazolate framework-9, ZIF-9). ZIF-9 is carbonized in an inert atmosphere to form nanoporous carbon–metallic-cobalt materials, followed by the subsequent thermal oxidation in air, yielding nanoporous carbon–cobalt-oxide hybrids. The resulting hybrid materials are evaluated as electrocatalysts for the oxygen-reduction reaction (ORR) and the oxygen-evolution reaction (OER) in a KOH electrolyte solution. The hybrid materials exhibit similar catalytic activity in the ORR to the benchmark, commercial, Pt/carbon black catalyst, and show better catalytic activity for the OER than the Pt-based catalyst.
Tremendous efforts in the development of nanoporous materials, including nanoporous carbon, have been made in recent years to create novel materials with desired structures and [a] Dr. W. Chaikittisilp, Dr. N. L. Torad, Dr. C. Li, Dr. S. Ishihara, Prof. Dr. K. Ariga, Prof. Dr. Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) Homepage: http:www.yamauchi-labo.com E-mail: [email protected] [email protected] [b] Dr. N. L. Torad, Prof. Dr. Y. Yamauchi Faculty of Science and Engineering, Waseda University 3–4–1 Okubo, Shinjuku, Tokyo 169-8555 (Japan) [c] Dr. M. Imura Optical and Electronic Materials Unit National Institute for Materials Science (NIMS) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) [d] Dr. N. Suzuki International Center for Young Scientists (ICYS) National Institute for Materials Science (NIMS) 1–2–1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) [e] Prof. Dr. K. Ariga, Prof. Dr. Y. Yamauchi Precursory Research for Embryonic Science and Technology (PRESTO) & Core Research for Evolutional Science and Technology (CREST) Japan Science and Technology Agency (JST) 1–1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304404. Chem. Eur. J. 2014, 20, 4217 – 4221
properties.[1] As a new class of nanoporous materials, metal–organic frameworks (MOFs) or porous coordination polymers represent fascinating properties and functions with modularly tunable structures assembled from metal cations and organic ligands.[2] In addition to the promising applications of MOFs, such as gas storage and separation, catalysis, sensing, and drug delivery, they have been demonstrated recently to be useful precursors for the synthesis of metal oxides with controlled particle sizes and morphologies,[3–5] and nanoporous carbon.[6–8] Because of the large variation of organic moieties in MOFs, highly nanoporous carbon, with excellent properties in gas adsorption, electrochemical capacitance, and sensing, can be achieved by thermal carbonization of several MOFs that serve as precursors or sacrificial templates in inert atmospheres. The procedure is followed by chemical treatments to remove any inorganic residuals from the carbon networks.[6–8] Xu et al. have reported MOF-derived nanoporous carbon for the first time.[7] Interestingly, MOF-derived nanoporous carbon can have exceptionally high surface areas and/or very uniform pore-size distributions.[6] However, the direct preparation of carbon–metaloxide hybrid materials from a MOF as a single precursor is still rare.[9] Recently, carbon–metal-oxide hybrid materials have become of particular interest for electrochemical applications, such as fuel cells and energy-storage devices. Oxygen-reduction reactions (ORRs) are important cathode reactions in fuel cells, metal–air batteries, and chlor-alkali electrolysis.[10] So far, platinum and alloys thereof have been found to be the most active electrocatalyst for ORRs. However, the cathode ORR is much slower than the anode oxidation reaction, thereby requiring a high loading of expensive Pt-based catalysts to accelerate the ORR.[11] This limits the commercialization of fuel cells, particularly in the automotive industry. In the course of searching for active and robust nonprecious metal catalysts for the ORR, metal–nitrogen complexes (where the metal is cobalt or iron) on carbon matrixes with M N4 (M = Co or Fe) catalytic active sites were found to show excellent performance for the ORR in acidic media.[12] Carbon-supported metal oxides, particularly cobalt-based oxides, have been found to be active ORR catalysts in alkali media.[13–15] Strong coupling between metal oxides and carbon matrixes has been suggested to enhance the electrocatalytic activity. Among several strategies, the direct nucleation, growth, and anchoring of metal oxides on the oxidized carbon atoms seem to be one of the most fasci-
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Communication nating ways to yield efficient carbon–metal-oxide hybrid electrocatalysts.[13] To the best of our knowledge, there are only a few reports in the literature on the utilization of MOFs, either as precursors[16] or as supports,[12d,e] for the preparation of electrocatalysts for the ORR in acidic media. In the former case, where MOFs were used as precursors, the direct thermal treatment of MOFs containing Co N bonds resulted in continuously connected carbon networks, thereby providing electrically conductive support for electrocatalysts.[16] In the latter reports, MOFs served as microporous hosts for catalyst precursors, that is, iron cations and bidentate N-donor ligands.[12d,e] Alternative to the above reports, we describe herein for the first time the preparation of nanoporous carbon–cobalt-oxide hybrid materials for the ORR in alkali media by direct conversion of a MOF as a single precursor. We anticipated that the resulting hybrids would possess a strong coupling between cobalt oxide and the carbon matrixes. Zeolitic imidazolate framework-9 (ZIF-9), which possesses a sodalite topology with hexagonal symmetry, is built from corner-sharing tetrahedral CoN4 units, in which the coordination bonds between the Co2 + cations and benzimidazolate anions is among the most stable for N-donor ligands.[17] In this study, ZIF-9 was selected as the cobalt-based MOF precursor due to its good thermal stability and high contents of carbon and nitrogen. ZIF-9 was solvothermally synthesized from cobalt nitrate and benzimidazole (see the Supporting Information, Figure S1). Conversion of ZIF-9 into carbon–cobalt-oxide hybrid materials was performed by two-step thermal treatments in an inert atmosphere and air, respectively (Figure 1). The resulting hybrid materials were designated as Z9-x-y, where x and y stand for the thermal treatment temperatures in the first and the second steps, respectively. Figure 2 a shows powder XRD patterns of the obtained hybrid materials after the first thermal treatment, indicating the presence of metallic cobalt in the materials. The XRD peaks become sharper as the treatment temperature increases, indicating an increase in the Co crystallite sizes. TEM images show that the Co nanoparticles are uniformly distributed over the entire area and that the sizes gradually increase with increas-
Figure 2. Powder XRD patterns (a,c) and nitrogen adsorption–desorption isotherms (b,d) of the samples obtained after the first thermal treatment in a nitrogen atmosphere (a,b) and after the subsequent treatment in air (c,d). The adsorption and desorption isotherms are presented as solid and open symbols, respectively.
ing treatment temperatures (see the Supporting Information, Figure S2). As assessed by nitrogen adsorption–desorption measurements (Figure 2 b), the materials are micro- and mesoporous with apparent specific Brunauer-Emmett-Teller (BET) surface areas of 415, 370, and 330 m2 g 1 for Z9-700-0, Z9-8000, and Z9-900-0 samples, respectively. Their total pore volumes (at P/Po = 0.99) are nearly identical, about 0.4 cm3 g 1. Thermogravimetric analysis (TGA) of the Z9-900-0 sample in air suggests that the cobalt particles undergo thermal oxidation as the sample weight increases with the maximum weight being at about 300 8C (see the Supporting Information, Figure S3). Figure 1. Schematic representation of the formation of carbon–cobalt-oxide hybrid materials through the twoAs a result, we first attempted to step thermal treatment of ZIF-9. Chem. Eur. J. 2014, 20, 4217 – 4221
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Communication oxidize the metallic cobalt particles in air at 300 8C. The XRD patterns of the samples obtained after the second thermal treatment clearly indicate the conversion of metallic cobalt to cobalt oxide (Co3O4, see the Supporting Information, Figure S4 a). The resulting hybrid materials, however, loose the porosities when the BET surface areas decrease to 50–70 m2 g 1. The sharp nitrogen sorption isotherms at high relative pressures (P/Po > 0.9) suggest the presence of large interparticular pores, due to the cobalt-oxide nanoparticle aggregates (see the Supporting Information, Figure S4 b). TGA results show that the residual weights (after 600 8C in air) are about 97, 94, and 82 wt % for the Z9-700-300, Z9-800-300, and Z9-900-300 samples, respectively, suggesting that the carbon networks decompose during the oxidative thermal treatments. TEM images of the Z9-x-300 samples (see the Supporting Information, Figure S5) also show that these samples are made of nanoparticle aggregates with interparticular pores. We hypothesized that the carbon networks are retained after oxidative thermal treatments at milder conditions. Therefore, the Z9-x-0 samples were subjected to the oxidative thermal treatment at 250 8C in air. As shown in Figure 2 c, the obtained samples clearly exhibit XRD peaks that arise from Co3O4 with some remaining cobalt metals. The coexistence of metallic cobalt was more markedly observed for the Z9-900-250 sample. In addition, the formation of CoO can be markedly seen in the case of the Z9-900-250 sample. As depicted in Figure 2 d, very interestingly, the nitrogen sorption isotherms indicate that after thermal treatment at 250 8C, the Z9-900-250 sample keeps the micro- and mesoporous structures, whereas the nanoporous carbon network of the Z9-700-250 and Z9-800-250 samples decompose, likely yielding the aggregates of cobalt-oxide nanoparticles, similar to the Z9-x-300 samples (see the Supporting Information, Figure S5). TEM images (see the Supporting Information, Figure S6) and the corresponding elemental mapping images (see the Supporting Information, Figure S7 and S8) also support the above claim. These results suggest that carbonization at 900 8C yields more thermally stable carbon networks than those carbonized at lower temperatures. The BET surface areas of Z9700-250, Z9-800-250, and Z9-900-250 were calculated to be 75, 100, and 280 m2 g 1, respectively. Total pore volumes (at P/Po = 0.99) are 0.34, 0.45, and 0.36 cm3 g 1, respectively. The TGA residual weights (after 600 8C in air) of Z9-700-250, Z9-800-250, and Z9-900-250 samples are about 90, 68, and 54 wt %, respectively, further supporting the retained carbon network in the Z9-900-250 sample. In addition, the TGA result suggests that a large amount of carbon still remains in the Z9-800-250 sample, although the porous carbon network is collapsed. Elemental analysis showed that our hybrid materials contain about 10 wt % of nitrogen. We expected that these nitrogendoped carbon networks should promote the ORR catalytic activities as the nitrogen-doped carbons were reported to catalyze the ORR and provide strong coupling between carbon networks and metal-oxide active species.[18] Further decrease of the oxidation temperature to 200 8C results in carbon hybrid materials with only small fractions of oxidized metallic cobalt. Chem. Eur. J. 2014, 20, 4217 – 4221
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The ORR catalytic activities of the resulting carbon–cobaltoxide hybrid materials were evaluated in an aqueous KOH (0.1 m) electrolyte solution through a rotating-disk electrode (RDE) method. RDE measurements were carried out to evaluate the ORR activity and kinetics in the O2-saturated KOH solution. Figure 3 a shows the ORR polarization curves recorded at a rotation rate of 2000 rpm with a scan rate of 5 mV s 1. Surprisingly, the Z9-800-250 sample exhibits better catalytic activity than the Z9-900-250 sample, although the Z9-800-250 sample has lower surface areas. The copresence of CoO and Co3O4 in Z9900-250 is not thought to give any negative effects on the ORR, because CoO-carbon hybrids and their Co3O4-carbon counterparts exhibit very similar, ORR half-wave potentials.[13d, 14c] The outperformance of Z9-800-250 over Z9-900-250 can therefore be attributed to the more complete conversion of metallic cobalt to cobalt oxide and the smaller sizes of cobalt oxide present in the carbon matrixes. Our previous report showed that ZIF-derived carbons obtained at 800– 1000 8C exhibit similar Raman spectra (i.e., similar relative ratios of the G band to the D band) and capacitances.[6c] The Z9-800-250 sample shows a half-wave potential that is about 55 mV more negative than that of the benchmark commercial Pt/carbon black (Pt/CB, 20 %) catalyst under the same conditions and outperformed the Pt/CB catalyst in the term of ORR current density at medium overpotentials; this is consistent with previous reports on similar materials.[13]
Figure 3. a) ORR polarization curves of RDE modified with Z9-700-250, Z9800-250, Z9-900-250, and Pt/CB. The plots are obtained in O2-saturated KOH (0.1 m) at a rotation rate of 2000 rpm with a scan rate of 5 mV s 1. b) ORR polarization curves of Z9-800-250 at different rotation rates in O2-saturated KOH (0.1 m) with a scan rate of 5 mV s 1. c) K–L plots of Z9-800-250, n represents the electron-transfer numbers. d) Polarization curves of glassy carbon electrodes modified with Z9-700-250, Z9-800-250, Z9-900-250, and Pt/CB in KOH (0.1 m).
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Communication To determine the ORR electron-transfer pathway, RDE measurements were performed at different rotation rates, giving the increasing current densities with increasing rotation rates (Figure 3 b). The electron-transfer numbers (n) were calculated from the slope of the Koutecky–Levich (K–L) plots (J 1 vs. w 1/2) on the basis of the K–L equation. Figure 3 c shows the K–L plot for Z9-800-250, which gives the electron transfer numbers of about 4, suggesting that the Z9-800-250 hybrid catalyzes the ORR mainly through a four-electron process. For comparison, the K–L plot at different potentials for commercial Pt/ CB is shown in the Supporting Information, Figure S9 a. Z9700-250 and Z9-900-250 exhibit slightly lower electron transfer numbers (see the Supporting Information, Figure S9 b). As expected, the Z9-x-300 materials exhibit poorer ORR activities with much more negative half-wave potentials than the Pt/CB catalyst (see the Supporting Information, Figure S10 a). The K–L plots shown in Figure S10 b–d, in the Supporting Information, give electron transfer numbers of 2.2, about 3.6, and about 3.5 for Z9-700-300, Z9-800-300, and Z9-900-300, respectively. Although the Z9-x-300 samples show more complete oxidation of metallic cobalt to cobalt oxide than the Z9-x-250 materials, the ORR activities of the Z9-x-300 samples are poorer; this is likely a result of the decomposition of the carbon matrixes. The OER or water oxidation, the reverse reaction of the ORR, is an anode reaction in an electrolysis cells, a half-cell reaction of water splitting (solar-fuel production) and is involved in the recharging process of metal-air batteries.[13a, 19] A bifunctional catalyst that can catalyze both the ORR and the OER is of particular importance as it can be employed in unitized regenerative fuel cells (URFCs), an energy-storage system for uninterrupted power supplies, solar-powered aircraft, satellites and microspacecrafts. URFCs can work as fuel cells and reversely as water electrolyzers to produce hydrogen and oxygen, with the ability to be coupled to intermittent renewable energy, such as wind or solar, to peak-shift electricity to the grid.[20] The resulting carbon–cobalt-oxide hybrid materials were therefore tested for the OER in KOH (0.1 m). As depicted in Figure 3 d, the Z9-700-250 and Z9-800-250 samples exhibit more negative OER onset potentials with higher current densities than Z9-900-250 and Pt/CB catalysts. These results indicate that the Z9-800-250 hybrid can act as an efficient electrocatalyst for both the ORR and the OER and hence may be utilized in the URFCs system. In summary, a direct, two-step thermal conversion of cobaltbased MOFs was performed in an inert atmosphere and in air, yielding nanoporous nitrogen-doped carbon–cobalt-oxide hybrid materials. Metallic cobalt nanoparticles embedded in the nanoporous carbon networks were achieved after the first thermal treatment (carbonization); subsequently, such cobalt nanoparticles were converted to cobalt oxides during the second thermal treatment (oxidation). These hybrid materials exhibited excellent catalytic activities for both oxygen reduction and evolution reactions, and accordingly may be a candidate catalyst for fuel-cells applications. Our simple thermal treatment is anticipated to be applicable to other structures of MOFs and the electrocatalytic activity of the resulting hybrids could be improved by fine tuning of crystallite size and morChem. Eur. J. 2014, 20, 4217 – 4221
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phology of the MOF precursors as well as the thermal-treatment conditions.
Experimental Section Synthesis of hybrid materials Carbon–cobalt-oxide hybrid materials were prepared by a two-step thermal treatment of ZIF-9. In the first step, well-ground ZIF-9 (0.5 g) was homogeneously dispersed in a ceramic boat that was then put into a tube furnace. The sample was exposed to a flow of nitrogen (ca. 45 mL min 1) at room temperature for about an hour and afterward the furnace was heated to the target carbonization temperature (700, 800, or 900 8C) at a heating rate of 5 8C min 1 with an intermediate step at 150 8C for 2 h. After reaching the target temperature, it was kept constant for 5 h and then cooled to room temperature naturally. The resultant black powder was collected for further uses. In the second step, the obtained black powder was thermally treated in air in a muffle oven. The sample was homogeneously dispersed in a ceramic disk. The ceramic disk was heated at the desired temperatures (250 or 300 8C) for 90 min with a heating rate of 1 8C min 1. The obtained product was collected for further characterizations.
Electrochemical test Voltammograms were obtained from an ALS/CHI 842BZ electrochemical analyzer at ambient temperature with a scan rate of 5 mV s 1. A platinum wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. A glassy carbon RDE, the working electrode, was coated with a catalyst ink (0.25 mg cm 2). The catalyst ink was prepared by dispersing catalyst (3 mg) in a Nafion water/ethanol solution (1 mL), which was prepared by adding Nafion (400 mg, 5 wt %) into a mixture of ethanol (12 mL) and water (8 mL). RDE measurements were conducted with a three-electrode electrochemical cell at different rotation rates while the electrolyte was continuously bubbled with oxygen. Before the RDE experiments, the aqueous KOH (0.1 m) electrolyte solution was bubbled with oxygen for at least an hour.
Keywords: cobalt oxide · electrocatalysts · mesoporous carbon · metal–organic frameworks · nanoporous carbon
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Received: November 10, 2013 Published online on March 12, 2014
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